1. Field of the Invention
This invention relates to floating offshore oil and gas production and drilling facilities in general and particularly with semisubmersible hull forms for deep and ultra deep water, wet tree and dry tree applications. This invention relates generally to floating offshore applications, including applications outside the offshore oil and gas industry.
2. Description of the Prior Art
Many substructures have been described in the prior art with applicability to offshore oil and gas drilling and production. The preferred substructure provides efficient and economical support to the drilling and production facilities, minimal motions to maximize the availability of drilling and production operations and to minimize damage to components both located on the substructure and hanging off the substructure, and requires few complex operations for fabrication, assembly and installation.
The following list provides a brief description of some of the existing substructures used for offshore applications.    1) Conventional Semisubmersible            A conventional semisubmersible hull form consists of a number of columns and pontoons as illustrated in FIG. 1A. A typical spread mooring system is employed for station keeping. The first conventional semisubmersible was built in 1975. One of the limitations of a conventional semisubmersible is that the substructure exhibits large motions in storm conditions which makes it not suitable for dry tree production applications and marginal for wet tree production applications using steel catenary risers.            2) Deep Draft Semi-submersible (DDS)            Similar to conventional semisubmersibles, a DDS is composed of a number of columns and pontoons with deeper design drafts to improve the platform motion characteristics, as shown in FIG. 1B. The design draft of a DDS is typically around 100 ft or more. A spread mooring system is often used for station keeping. The DDS has been built for wet tree production applications. However, no DDS has been installed for dry tree production applications. The DDS has improved motions over a conventional semisubmersible, but the motions still limit applicability and negatively impact drilling and production operations.            3) Adjustable-Base Semisubmersible (ABS)            The adjustable-base semisubmersible is another hull form concept that has been proposed to further improve the platform motion characteristics to satisfy the requirements of dry tree production. A typical ABS concept is shown in FIG. 1C. The ABS design draft is typically about 250 ft. The ABS employs large moving components that are subjected to significant forces from wind, waves and current. The complexity of large moving components reduces the efficiency (weight and cost) of these concepts.            4) Truss Spar            The truss spar is a deep draft floating platform with a hull length of around 600 ft or more. The spar hull consists of three portions: 1) an upper buoyant structure to provide the necessary buoyancy, 2) a keel structure that holds solid ballast for improved stability and motions, and 3) a truss structure that rigidly connects the keel structure to the upper buoyant structure. A sketch showing a typical truss spar is illustrated in FIG. 1D. The truss spar is suitable for wet and dry tree offshore oil and gas production but requires offshore assembly due to the very deep draft.            5) Tension Leg Platform (TLP)            The TLP is another substructure with a history of both wet tree and dry tree production applications. A TLP hull form is composed of a number of vertical columns and horizontal pontoons vertically moored to the sea bed by a number of tendons as shown in FIG. 1E. Due to the vertical restraint of the tendons, the TLP virtually has no vertical dynamic movement. However, the tendon system significantly reduces the efficiency of the structure in very deep water, especially for larger payloads.        
Although there are several existing substructure designs that are used for offshore applications, each of the existing designs has limitations that increase complexity or reduce efficiency, thereby increasing the cost and risk associated with implementation. A partial listing of the undesirable characteristics of the existing technology for wet tree and dry tree production facilities for offshore application is given below.    1) Conventional Semisubmersible—While conventional semisubmersibles have acceptable motion responses in normal weather, their motion responses during severe storm conditions are typically excessive and unacceptable for some applications. Specifically, vertical motions (heave) are too large for dry tree operations and limit operability for drilling operations.    2) Deep Draft Semisubmersible—Similar to the conventional semisubmersible, vertical motions (heave) are too large for dry tree operations using existing riser tensioning equipment.    3) Conventional Semisubmersible, Deep Draft Semisubmersible, and TLP—Surge motion can generate unacceptable fatigue damage for steel catenary risers, particularly those with large diameter and/or high pressure, high temperature and sour service application.    4) Deep Draft Semisubmersible—Design efficiency is limited by the conflicting requirements of minimizing deck span between columns versus in-place and pre-service stability requirements, which require increased distance between columns.    5) Adjustable-Base Semisubmersible—The connection design between the main hull and the extended base structure requires complex and unproven adjustable mechanisms which must withstand large loads, fatigue loads, and long platform life.    6) Truss Spar—High cost associated with construction, transportation and offshore integration. Deck structure with production facilities must be installed offshore using a very limited class of heavy-lift construction vessels and operations that are subject to potential delays due to weather sensitive operations.    7) TLP—High cost associated with vertical mooring system for ultra deepwater applications.
Suitable deepwater floating production platforms for the offshore oil industry are needed to permit the economical development of petroleum reserves in the increasingly deep waters in which fields are being located.
Prior art for improvements to the semisubmersible substructure include the addition of heave damping plates (Sarwe, U.S. Pat. No. 4,823,719), the use of multiple structures that must be joined offshore (Wetch, U.S. Pat. No. 6,666,624), movable components that must be extended by jacking or ballasting (Merchant, et al, U.S. Pat. No. 7,219,615) combinations of semisubmersible substructures with tension leg substructures using complex guides and mechanisms (Goldman, U.S. Pat. No. 4,995,762), introduction of a column belt in the vicinity of and across the water surface (Yamashita et al., U.S. Pat. No. 4,987,846), or motion reduction by increasing damping through prescribed pontoon geometry (Bowes, U.S. Pat. No. 4,909,174). Another semisubmersible concept (Wybro, U.S. Pat. No. 7,140,317) seeks to simplify construction by using a unitized central-pontoon structure located inboard of the columns. This central-pontoon concept reduces support spans for the pontoon but does not improve support of the deck structure. The central-pontoon concept also discloses vertical columns of rectangular cross section that have the major axis oriented radially outward from the center of the hull and therefore reduces the support spans and cantilevers of the deck structure. However, this feature requires elongating the column rectangular cross section to reduce the deck support span and there are practical limits to this approach. The present invention instead provides column pairs that can be square, rectangular or circular and still reduce the deck support spans as disclosed further in this specification.
The primary objective is to develop an offshore substructure with motions suitable for dry tree support or improved drilling operations. All of these and similar proposals for semisubmersible substructures suffer from one or more of the limitations provided above, either not achieving the desired motions or being overly complex such that fabrication and installation carry too much cost and/or risk. Economic constraints require that the production platform have an efficient design that is installable in a completed condition on location in deep water at an affordable cost. The current platform designs, while adequate in some respects, are sufficiently expensive that many production fields are not developed.
The objectives of the present invention are    1) To present a semisubmersible substructure that has the ability to de-couple constraints on column spacing due to deck support requirements from the constraints on column spacing due to overall platform stability, which will subsequently allow the designer to minimize platform motion responses by optimizing the overall platform configuration;    2) To present a semisubmersible substructure that has sufficiently small motion characteristics suitable for both wet tree and dry tree production applications, including applications utilizing top-tensioned risers;    3) To present a semisubmersible substructure that has sufficiently small surge motion characteristics to be compatible with large diameter steel catenary risers, particularly in high pressure, high temperature and/or sour service design conditions and even for water depths less than 4,000 ft;    4) To present a semisubmersible substructure that can be fully integrated quayside prior to offshore installation to minimize the cost and risk associated with offshore construction and commissioning operations;    5) To present a semisubmersible substructure that is composed of conventional structural components and concepts and without the use of complicated adjustable mechanisms;    6) To present a semisubmersible substructure that utilizes conventional constructability concepts and draft requirements compared to truss spars and deep-draft semisubmersibles;    7) To present a semisubmersible substructure that accommodates a conventional center well bay design and conventional drilling and riser support equipment for reliable drilling and riser operations; and    8) To present a semisubmersible substructure with virtually no limiting water depth constraints and which therefore can be employed in ultra deep water depths of 10,000 ft or beyond.